Well for transporting hot fluids through a permafrost zone

ABSTRACT

A well adapted to transport hot fluids through a permafrost zone comprising a surface conductor, a short section of refrigerated casing adapted to support the upper section of the well without melting of the surrounding permafrost, a surface casing extending through the permafrost, production casing and production tubing. All casing strings are supported in the well so that the upper sections are in compression. Pressure integrity between casing strings is maintained by packing glands in the well head. Also, a short section of casing can be provided in the annulus exterior of the surface casing to provide a separately enclosed refrigeration reservoir, or to provide for circulation of refrigerant.

[ Nov. 28, 1972 United States Patent Rardin [5 WELL FOR TRANSPORTING HOTOTHER PUBLICATIONS FLUIDS THROUGH A PERMAFROST ZONE Alaskan Completionswill be Complicated, World Oil, January 1970, Page 85.

m5 5 ..D me m mu 0 mm A we 3 2w... mm mm? m m m n v. m M @C W w. Rm Um mm m [22] Filed: Nov. 6, 1970 ABSTRACT 21 A 1. N 87 9 PP ,44 A welladapted to transport hot fluids throu gh a permafrost zone comprising asurface conductor, a short section of refrigerated casing adapted to su[52] U.S. Cl............ ......166/302, 166/D1G. l, 166/57 pport the[51] Int. Cl. 43/24 upper section of the well without melting of thesur- [58] Field of Search....................166/302, 57, DIG. 1 r n ingp f surface ing en ing rrnaf through the pe rost, production casing andproduction tubing. All casing strings are supported in pper sections arein compression.

[56] References Cited the well so that the u UNITED STATES PATENTSPressure integrity between casing strings is maintained 2,772,73712/1956 Bond et a1. ..................l66/57 Yj t g' i 8 3,142,336 66/57g pl'OVl e in e annu us ex erior 3,160,208 166/57 3,405,763 166/2083,498,381 3,561,531

parately enclosed geration reservoir, or to provide for circulation ofrefrigerant.

of the surface casing to provide a se refri 15 Claims, 2 Drawing Figures7/1964 Doscher 12/1964 Jorda........................... 10/1968 Pitts et3/1970 Earlougher, Jr.............166/57 2/1971Miller...................l66/DIG. 1

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WELL FOR TRANSPORTING HOT FLUIDS THROUGH A PERMAFROST ZONE Thisinvention relates to wells for recovering earth fluids, and moreparticularly to wells penetrating a permafrost zone and to a method forcompleting such wells.

Permafrost is perennially frozen ground found in the Artic regions. Thepermafrost zone contains some layers of gravel free of ice known as drypermafrost, but the bulk of the zone is composed of rocks or ofunconsolidated aggregates of sand, silt. and gravel in which theinterstitial water is frozen to ice. Permafrost is formed by thespreading downward of the low temperatures found at the surface in theArtic winters, particularly in regions of low snowfall. There isseasonal thawing and freezing at the surface, but thawing rarelypenetrates more than 18 inches where the permafrost is protectedbytundra or vegetation. The downward spread of freezing temperatures toform permafrost continues until an equilibrium is reached with the heatflow from the earths interior. The thickness of the permafrost zonevaries with latitude and with particular geographic location. Permafrostis not believed to exist under most deep large Artic lakes or under theArtic Ocean.

Permafrost generally has sufficient strength to support oil explorationand recovery operations, so long as it remains frozen. However, whenmelted, the permafrost shrinks and subsides, causing a downdrag force ona well casing in the permafrost zone. Drilling operations can generallybe conducted without causing any substantial melting of the permafrost.However, melting of the permafrost is experienced around wellsconducting hot fluids through the permafrost zone. While various methodsof completing wells in the permafrost to permit production of hot oilfrom lower zones have been proposed, none of these techniques have beencompletely satisfactory. Thus, need exists for a method of completingwells in the permafrost that will permit the production of hot oil orinjection of hot fluids without damage to the well from subsidence orfailure of the supporting earth around the well.

Accordingly, a primary object of this invention is to provide atechnique for completing a well in a permafrost zone. Another object ofthe invention is to provide a well for transporting hot fluids through apermafrost zone that remains structurally supported by the permafrost.Yet another object of the invention is to provide a method forcompleting a well in a permafrost zone.

The manner of accomplishing the foregoing objects as well as furtherobjects and advantages of the invention will be apparent from thefollowing description taken in conjunction with the drawings, whereinlike numerals refer to corresponding parts, and in which:

FIG. I. is a schematic vertical sectional view illustrating the wellassembly of this invention installed in a percomprised of surfaceconductor 14 cemented in the upper strata of permafrost zone 10 withcement 16. The drilling rig is often conveniently set on pilingprefrozen in the permafrost at a depth of about 20 feet. Surfaceconductor 14 is run to eliminate the possibility of a wash out or severemelting around the piling and cellar area directly adjacent to the wellbore. Setting the surface conductor at a depth below the bottom of thepiling eliminates the possibility of melting or wash out around thepiling and loss of adequate rig support. Typically, surface conductor 14is comprised of a length of relatively large diameter pipe, such as30-inch diameter casing, 20 to feet in length, and preferably about 40feet in length.

Casing 20 is the main support string for the balance of casing stringsand provides initial blowout control protection until casing isinstalled through the permafrost. Fluted subsea hanger 22 is run at thebottom of casing 20 and provides the point at which the surface casingstring is landed. Casing 20 is cemented in place by means of cement 24.Casing 20 extends into the permafrost zone sufiiciently to providesupport for itself and the other casing strings, the permafrostsurrounding this section of casing being maintained permanently frozenover substantially its entire length to provide the necessary supportfor the well. In a typical installation, casing 20 extends to a depth of350 feet and the permafrost is maintained frozen to a depth of at least300 feet by means refrigeration. Maintaining the permafrost frozen to adepth of 300 feet should provide adequate support for the well, and anythawing below this point is considered harmless.

Casing 20 serves as the outer wall of the refrigeration reservoir forthe phase change refrigeration system, with casing 30 serving as theinner wall. Annulus 26 between casing 20 and casing 30 is sealed withpacking in the well head and by hookwall packer 28 run at the bottom ofcasing 30, the packer providing support for casing 30 and sealing thebottom of annulus 26. When filled with refrigerant this annulus willserve as the heat sink or evaporator side of the cooling system. Casing30 should remain at a substantially constant temperature created by therefrigerant, and will be standing in compression supported by thehookwall packer, with packing only being installed in the surface casinghead. Refrigerant is added to annulus 26 through a small diameterconduit 32 strapped on the exterior of casing 30.

Surface casing 40 is the primary pressure protection string, and is rundeep enough for protection from possible shallow zones and deep enoughto cover the entire permafrost section. The lower section of casing 40and subsea type packoff and re-entry assembly 22 is run on the casingalong with a sub-sea landing mandrel. The landing mandrel typically hasan outside diameter larger than casing 30 and, therefore, the lowersection of casing 40 with the sub-sea hanger is run on drill pipe,

hung from the shoe and cemented with cement 44 prior to installation ofcasing 30. The top section of casing 40 with the re-entry assembly isrun following installation of casing string 30. The use of the packoffand reentry assembly also allows a means for displacing mud and cementfrom the well bore with diesel oil. Displace ment of mud from all wellannuli adjacent permafrost zones is necessary to prevent freezing of themud which may fracture the well casing.

Casing 50 is run as the production string and will be subjected todirect zone pressure. It will be suspended from subsea type hanger andmandrel assembly 52 at the bottom of casing 40, or alternatively from apoint adjacent hanger 22. Casing 50 will be provided with a DV cementer54, or other means for change over to diesel in the subsea hanger, at apoint above the bottom of casing 40, and a DV cementer at such otherlocations as required for proper zone separation and cement coverage.Change over from mud and cement to diesel oil above subsea hanger 52will prevent freezing of fluids in casings 40 and 50 opposite thepermafrost. As in the case of casing 40, the upper section of casing 50above hanger 52 will be supported in compression, with a packing glandinstalled around the casing at the surface and flanged to casing 40.This will allow differential expansion and contraction between casing 40and casing 50 without creating excessive stress.

The well is completed in conventional manner by extending casing 50through h the productive zones and perforating at selected intervals, orby terminating the casing above the productive zones and hanging a linerin these zones that is pre-slotted or perforated. Production tubing 60handles production flow from the producingzones to the surface, or flowfrom the surface to the producing zones if the well is to be used as aninjector. A packer, not shown, will be run in and set on tubing 60 abovethe perforations, and the tubing will be landed on subsea type hangerand mandrel assembly 62 located adjacent hanger and mandrel assembly 52.Diesel oil is circulated as the packer fluid in the annulus between theproduction tubing and casing 50. Tubing 60 is provided with insulation64 extending through the permafrost zone to reduce the transfer of heatinto the permafrost and to minimize the refrigeration load. Anyconvenient means of insulating tubing 60 can be employed, such asstrapping half cylinders of 2-inch thick polyurethane on the tubing inthe permafrost zone.

Centralizers, such as the centralizers 34, should be provided tomaintain casings 30, 40 and 50 centered in the permafrost zone. Also,centralizers can be provided for tubing 60 to protect insulation 64.

The well of this invention is. completed with a well head assembly thatpermits independent vertical expansion and contraction of the variouscasing strings without creating excessive stress and provides pressureintegrity between casing strings. Referring more specifically to FIG. 2,casing 20 is fitted with a slip and packing seal type well head 70having an inner landing and seal assembly 92 to accommodate casing 30.Casing 30 is packed off in the well head, but the slips are not used.Casing pack-off flange 72 having a steamtype packing assembly 94 isinstalled around casing 40. The stub of casing 40 projecting abovepack-off assembly 72 is fitted with a slip and packing assembly typewell head 74 having an inner packing seal 96. Casing pack-off flange 76having a steam-type packing assembly 98 is installed around casing 50and slip and packing type tubing head 80 is installed on the upwardlyprojecting stub of casing 50. Tubing 60 is landed in a donut type hangerin the tubing head and block-type Christmas-tree 82 is mounted on tubinghead 80. The landing slips in well head 74 and tubing head 80 are notutilized.

Conduit 84 connects the wing valve of well head 70 to the suction ofcompressor 86. The discharge of compressor 86 is connected to condenser88 and to well head by conduit 90, which in turn is connected to conduit32 internally in well head 70. Condenser 88 is preferably air cooled toavoid freezing problems associated with water coolers during the wintermonths. In operation, hot fluids are withdrawn from the lower productivestrata or are injected into these strata through tubing 60. While theflow of heat from the hot fluid into the permafrost zone is minimized byinsulation 64, nevertheless suflicient heat is transferred to thepermafrost below the refrigerated section of casing 20 to cause meltingof the permafrost in the region 18 surrounding the well bore. Melting ofthe permafrost in the region 18 and the resulting subsidence andsettling is not considered a problem since the upper section-of the wellis firmly supported by the permanently frozen permafrost above thisregion. In this manner, it is not necessary to refrigerate the wellthrough the entire permafrost zone, but only a sufficient length of thewell need be refrigerated to provide the necessary support.

With the aforedescribed well casing system, all casing and tubingstrings are supported internally within the well and the upper sectionof each casing and tubing string above its respective landing shoe isstanding free. Excepting for casing 30, the upper section of each casingstring is free to move independently of support string 20, thus avoidingexcessive tensile and compressive forces created by expansion andcontraction caused by temperature changes. Pressure integrity of eachcasing string is provided by the steam-type packing glands installedaround each individual casing string and flanged to the casing headinstalled on the next outer casing string. Casing 30, which is not asupport or protective string, is at substantially the same temperatureas casing 20 and will thermally expand at about the same rate as casing20. Therefore, it is not necessary to provide for any significantdifferential expansion between casing 30 and casing 20.

While any suitable refrigeration system can be employed to maintain thepermafrost surrounding casing 20 frozen, a phase change system employingthe vaporization of a refrigerant for heat removal is preferred. Forexample, a liquid refrigerant such as propane, butane, or otherhydrocarbon, or a halogenated hydrocarbon refrigerant, or ammonia isintroduced into annulus 26 by means of conduit 32. A reservoir of therefrigerant is maintained in the annulus and, as heat is absorbed, therefrigerant is vaporized. The vapors pass upwardly in the annulus andare withdrawn through conduit 84, compressed, condensed and returned tothe annulus.

In another embodiment of the invention, packer 28 at the bottom ofcasing 30 is deleted, or other means of communication provided betweenthe annulus 26 and the annulus on the interior of casing 30. Refrigerantis then circulated downwardly through annulus 26 between casing 20 andcasing 30 and upwardly through the annulus between casing 30 and surfacecasing 40. Also, in a non-preferred embodiment of the invention, casing30 can be omitted and the annulus between casing 20 and surface casing40 used as the refrigeration reservoir.

While it is to be recognized that considerable latitude and flexibilityis available in the design of the various casing strings to meet theparticular circumstances and conditions encountered, the followingcasing schedule represents a suitable design for a completionencountered in many permafrost regions.

Diameter, Pipe Weight, Depth, Casing Inches Pounds/Foot Feet Surfaceconductor 30 l 19 40 Support string 20 94 350 Intermediate string 16 75300 Surface string 13% 61 2500 Production string 9% 44.3 10,000:

Also, it is to be recognized that additional casing strings can be runbelow the surface strings where required by local conditions. Theseadditional casing strings would be landed in the well and packed off inthe well head in similar manner to provide free expansion at the top ofthe well.

A well in accordance with this invention can be drilled employinggenerally conventional Artic drilling practices modified in the mannerhereinafter described so as to provide the desired installation.

The well is spudded in by drilling a large diameter hole with anauger-type drill to a depth below the support piles of the drilling rig,e.g., to a depth of 20 to 60 feet. Surface conductor 14 is run into thehole and stood on the bottom and cement l6 placed around the conductor.

Next, a smaller diameter hole is drilled to a sufficient depth toprovide support for the well, e.g., to about 350 feet, and casing 20 isrun into the hole and cemented in the proper location. Well head 70 isinstalled on casing 20.

Next, an even smaller diameter hole is drilled to a point below thepermafrost zone. The lower part of casing 40 is set in the hole on drillpipe and hung from hanger 22 at the bottom of casing 20. Casing 40 iscemented in the hole with cement 44. Casing 30 is run into the hole andpacker 28 set at the desired depth. This casing is packed off in wellhead 70 and refrigeration conduit 32 connected to an outlet in thecasing head. The upper section of casing 40 is run into the hole andconnected to the lower section of casing 40 previously placed in thehole. Casing packoff flange 72 having a steam-type packing assembly isinstalled on casing 40 and made up to well head 70. Well head 74 isinstalled on casing 40.

The. hole is then drilled to the final depth and production casing 50 isrun in and hung from hanger 52 at the bottom of casing 40. Casing 50 iscemented above the bottom of casing 40 with cement 56. Casing packoffflange 76 having a steam-type packing assembly is installed on casing 50and made up to casing head 74. Tubing head 80 is installed on casing 50.The well is perforated or otherwise completed for production orinjection service and tubing 60 having insulation in the permafrost zoneis run into the well and hung from hanger 62. Christmas-tre 82 ismounted on tubing head 80 and the refrigeration system installed andplaced in service. The well is now placed on production or injectionstarted, depending on the service in which the well is to be used.

It is preferred that all cementing in the permafrost zone be done withhigh alumina cements of the socalled fondu type such as that marketedunder the trademark Ciment Fondu, or with 50/50 mixtures of ash andfondu cement. Also, cement and mud should be circulated out of allcasing in the permafrost zone with diesel oil to prevent freezing andpossible rupture of the casing.

This invention is further illustrated by the following example which isillustrative of a specific mode of practicing the invention and is notintended as limiting the scope of the invention as defined by theappended claims.

The well is spudded in by drilling a 36-inch hole to 40 feet with anauger type drill. 40 feet of 30-inch casing is run into the hole andstood on the bottom. Fondu cement is poured around the casing to groundlevel.

Next, 26-inch hole is drilled to 350 feet with a conventional rotary bitusing refrigerated mud to prevent thawing of the permafrost. 20-inch,94-pound casing is fitted on the bottom with a drill pipe stab-in shoe,and a 20-inch X 13-%-inch subsea fluted casing hanger is installed atthe first joint above the shoe. The casing is run and hung in the rotarytable so that the shoe is at the proper depth. Drill pipe is then runand stabbed into the shoe. Fondu cement is pumped down the drill pipeand returned to the surface up the annulus on the exterior of the20-inch casing. The drill pipe is pulled and a 20-inch slip and packingseal type of well head fitted with an inner landing to accommodate16-inch casing is installed. A 20-inch pack-off type of blowoutpreventer and well head flange is then installed.

Next, l7- /z-inch hole is drilled to approximately 2,500 feet 13-%-inch,61 pound casing is fitted with a 20-inch X l3-%-inch landing mandrel tosuspend the l3-%-inch casing from the previously installed 20-inch Xl3-%-inch fluted hanger. A subsea packoff and reentry assembly isinstalled on top of the hanger mandrel and the inner top sleeve of thepackoff and re-entry assembly is subbed to a drill pipe. A conventionalfloat shoe and a l3-%-inch X 9-%-inch landing mandrel is installed onthe bottom joint of l3-%-inch casing and a float collar on the firstjoint above the float shoe. A liner type plug is suspended in thel3-%-inch casing on the bottom of the drill pipe running in sub. Thecasing is then lowered on drill pipe and landed in the 20-inch l3-%-inchhanger and cemented to the hanger with fondu/ash 50/50 cement followedby fondu cement. The floats are tested for positive check andimmediately thereafter the drill pipe is released from the subseapackoff and re-entry assembly and any cement around the mandrel andhanger is circulated out to the surface. The drilling mud is displacedwith diesel oil and the drill pipe sub and top sleeve of the packoff andre-entry assembly is pulled, out of the hole.

Approximately 300 feet of l6-inch, pound casing having a l6-inch 20-inchmechanical set packer on the bottom and a l-inch circulating stringstrapped to the exterior is run and hung in the 20-inch well head. Thepacker is set, tested and the l6-inch casing packed off in the wellhead. The slip assembly in the 20-inch well head is not used. The l-inchcirculating string is connected to the outlet of the casing head.

The balance (approximately 305 feet) of 13-%-inch casing with a subseatop packoff and re-entry sleeve on the bottom joint is run into the welland stabbed into and secured to the subsea packoff and re-entry assemblyinstalled at the top of the previously run 13-% inch casing. The casingstring is tested for leaks, the 20- inch blowout preventer removed and a20-inch, 2000 pound X l3-%-inch, 2000 pound-casing packoff flange havinga steam-type packing assembly installed around the 13-%-inch casing. Thel3-%-inch casing is cut off one foot above the packing gland and a12-inch, 3000 pound slip and packing seal type well head is installed onthe l3-%-inch stub. A 12-inch blowout preventor is installed and tested.

Next, l2-V4-inch hole is drilled 10,000 feet and 9-%- inch casing run.The casing has a cement float shoe on the bottom, a float collar twojoints above the bottom and DV cementers at intervals. The top DVcementer is installed at 2100 feet (just below the bottom of thepermafrost). A l3-%-inch X 9-%-inch subsea hanger is installed justbelow the top DV cementer to land the casing. Alternatively, means forcirculating the well can be provided in the subsea hanger and the top DVcementer deleted. The 9-%-inch casing is cemented so that the top ofthecement is brought high enough for definite tie-in to the l3-%-inchcasing. The top DV cementer is then opened and the cement circulated outof the 9-%-inch annulus above the l3-%-inch X 9-%-inch landing mandrelwith mud. The cement is allowed to harden for about 10 hours, then themud is circulated out the top DV cementer with diesel oil and the DVcementer is closed. The blowout preventer is removed and a casingpackoff flange with steam-type packing assembly is then installed aroundthe 9-%-inch casing and the casing is cut off one foot above the packinggland. A 10-inch, 5000 pound slip and packing type tubing head isinstalled on the 9-%-inch casing stub.

A blowout preventer and test connections are installed on the 10-inchtubing head. The well is perforated and prepared for production.3-%-inch tubing insulated in the permafrost zone is run on a singlehydroset packer and landed on the l3-%-inch X 9-%- inchsubsea hanger. Apacking gland is provided in the tubing head to seal the tubing. AChristmas-tree is mounted on the tubing head and the refrigerationequipment installed and connected to the -inch by 16-inch annulus.

Diesel oil is pumped down the annulus and out the tubing to-displace themud. The hydroset packer is set. The wing valve in the well head betweenthe 16 and 20- inch casing strings is opened. Liquid butane is pumpeddown the l-inch stinger and up the annulus to displace the diesel andprepare the refrigeration system for operation. The well is placed onproduction and the refrigeration system is placed in service andadjusted to maintain the permafrost zone at a constant temperature ofabout 14F.

Various embodiments and modifications of this invention have beendescribed in the foregoing description and drawings, and furthermodifications will be apparent to those skilled in the art. Suchmodifications are included within the scope of this invention as definedby the following claims.

Having now described our invention, we claim:

1. A well for conducting hot fluids through a permafrost zone, whichcomprises:

a surface conductor cemented in the permafrost;

a first casing placed concentrically within said surface conductor andcemented therein, said casing extending sufficiently into the permafrostto provide support for the upper section of the well;

a surface casing placed concentrically Within said first casing andextending downwardly through the permafrost zone, said surface casingbeing sup ported from the bottom of said first casing and said casingbeing cemented below the bottom of said first casing;

means to refrigerate the annulus between said first casing and saidsurface casing;

a production casing placed concentrically within said surface casing andextending downwardly to the producing strata, said casing beingsupported from said surface casing at a point within the well; and

production tubing to convey fluids between the producing strata and thesurface or vice versa, said tubing being insulated through thepermafrost zone and said tubing being supported from the productioncasing within said well.

2. The apparatus defined in claim 1 including a second concentricintermediate casing placed between said first casing and said surfacecasing and terminating above the bottom of said first casing, saidsecond casing being supported from said first casing at a point near itsbottom.

3. The apparatus defined in claim 2 wherein the annulus between saidfirst and said second casings is closed at its bottom by a packer andincluding a small diameter conduit fastened to said second casing andextending substantially its length to introduce refrigerant into saidannulus, and means to withdraw vaporized refrigerant from the top ofsaid annulus.

4. The apparatus defined in claim 2 ,wherein the annulus between saidfirst and said second casings is open at the bottom and whereinrefrigerant is circulated down the annulus between said first and saidsecond casings and up the annulus between said second casing and saidproduction casing.

5. The apparatus defined in claim 2 including a well head comprisingmeans to seal the annulus between said first and said second casings, apacking gland to seal the annulus between said second casing and saidsurface casing, a packing gland to seal the annulus between said surfacecasing-and said production casing, a tubing head and a Christmas-tree.

6. A well for conducting hot fluids through a permafrost zone, whichcomprises:

a large-diameter surface conductor cemented in the permafrost;

a first casing placed concentrically within said surface conductor andcemented therein, said casing extending sufficiently into the permafrostto provide support for the well in the permafrost zone;

a second casing placed concentrically within said first casing andtenninating above the bottom of said first casing, said second casingbeing supported from said first casing at a point near its bottom; I

a surface casing placed concentrically within said second casing andextending downwardly through the permafrost zone, said surface casingbeing sup ported from the bottom of said first casing and said surfacecasing being cemented below the bottom of said first casing;

a production casing placed concentrically within said surface casing andextending downwardly to the producing strata, said casing beingsupported from said surface casing at a point within the well and saidcasing being cemented below said surface casing;

production tubing to convey fluids from the producing strata to thesurface or vice versa, said tubing being insulated through thepermafrost zone and said tubing being supported from the productioncasing within said well; and

means for refrigerating the interior of said first casing.

7. The apparatus defined in claim 6 wherein the annulus between saidfirst and said second casings is closed at its bottom by a packer andwherein said means to refrigerate said first casing includes a smalldiameter conduit fastened to said second casing and extendingsubstantially its length to introduce refrigerant into said annulus, andmeans to withdraw vaporized refrigerant from the top of said annulus.

8. The apparatus defined in claim 6 wherein the annulus between saidfirst and said second casings is open at the bottom and wherein saidmeans for refrigerating said first casing includes means for circulatingrefrigerant down the annulus between said first and said second casingsand up the annulus between said second casing and said surface casing.

9. The apparatus defined in claim 6 wherein said first casing extendsinto said permafrost to a depth of about 350 feet.

10. The apparatus defined in claim 6 including a casing head on saidfirst casing, means to seal the annulus between said first and saidsecond casings, a packing gland to seal the annulus between said secondcasing and said surface casing, a casing head on said surface casing, apacking gland to seal the annulus between said surface casing and saidproduction casing, a tubing head on said production casing, and aChristmas-tree mounted on said tubing head.

11. A well for conducting hot fluids through a permafrost zone, whichcomprises:

a large-diameter surface conductor cemented in the permafrost;

a first casing placed concentrically within said surface conductor andcemented therein, said casing extending sufficiently into the permafrostto provide support for the well in the permafrost zone; second casingplaced concentrically within said first casing and terminating above thebottom of said first casing, said second casing being -supported fromsaid first casing at a point near its bottom;

a casing head on said first casing including means to seal the annulusbetween said first casing and said second casing; surface casing placedconcentrically within said second casing and extending downwardlythrough the permafrost zone, said surface casing being supported fromthe bottom of said first casing and said surface casing being cementedbelow the bottom of said first casing; a packing gland to seal theannulus between said second casing and said surface casing;

a casing head installed at the top of said surface casa productioncasing placed concentrically within said surface casing and extendingdownwardly to the said tubing being supported from the production Icasing within said well;

a Christmas-tree mounted on said tubing head; and

means for refrigerating the interior of said first cas- 12. Theapparatus defined in claim 6 wherein the annulus between said first andsaid second casings is closed at its bottom by a packer and wherein saidmeans to refrigerate said first casing includes a small diameter conduitfastened to said second casing and extending substantially its length tointroduce refrigerant into said annulus, and means to withdraw vaporizedrefrigerant from the top of said annulus.

13. The apparatus defined in claim 6 wherein the annulus between saidfirst and said second casings is open at the bottom and wherein saidmeans for refrigerating said first casing includes means for circulatingrefrigerant down the annulus between said first and said second casingsand up the annulus between said second casing and said surface casing.

14. The apparatus defined in claim 11 wherein said first casing extendsinto said permafrost to a depth of about 350 feet.

15. A method for drilling a well through a permafrost zone, whichcomprises:

drilling a large diameter hole into said permafrost to a depth below thebottom of the piles supporting the drilling rig;

placing a surface conductor pipe in said hole and cementing saidconductor therein;

drilling a smaller diameter hole in said permafrost to a depthsufficient to support the well in the permafrost;

placing a first casing in said hole and cementing said first casing tothe surface; drilling an even smaller diameter hole to a depth below thebottom of the permafrost;

placing the lower section of a surface casing in said hole, supportingsaid surface casing from the bottom of said first casing, and cementingsaid surface casing above the bottom of said first casing; placing asecond casing in said first casing and supporting said second casingfrom said first casing; placing the upper section of said surface casingin said well;

drilling an even smaller diameter hole through the producing zones;

placing a production casing in said well and supporting said productioncasing from said surface casrefrigerating the annulus between said firstand said second casings; and placing said well in service.

1. A well for conducting hot fluids through a permafrost zone, whichcomprises: a surface conductor cemented in the permafrost; a firstcasing placed concentrically within said surface conductor and cementedtherein, said casing extending sufficiently into the permafrost toprovide support for the upper section of the well; a surface casingplaced concentrically within said first casing and extending downwardlythrough the permafrost zone, said surface casing being supported fromthe bottom of said first casing and said casing being cemented below thebottom of said first casing; means to refrigerate the annulus betweensaid first casing and said surface casing; a production casing placedconcentrically within said surface casing and extending downwardly tothe producing strata, said casing being supported from said surfacecasing at a point within the well; and production tubing to conveyfluids between the producing strata and the surface or vice versa, saidtubing being insulated through the permafrost zone and said tubing beingsupported from the production casing within said well.
 2. The apparatusdefined in claim 1 including a second concentric intermediate casingplaced between said first casing and said surface casing and terminatingabove the bottom of said first casing, said second casing beingsupported from said first casing at a point near its bottom.
 3. Theapparatus defined in claim 2 wherein the annulus between said first andsaid second casings is closed at its bottom by a packer and including asmall diameter conduit fastened to said second casing and extendingsubstantially its length to introduce refrigerant into said annulus, andmeans to withdraw vaporized refrigerant from the top of said annulus. 4.The apparatus defined in claim 2 wherein the annulus between said firstand said second casings is open at the bottom and wherein refrigerant iscirculated down the annulus between said first and said second casingsand up the annulus between said second casing and said productioncasing.
 5. The apparatus defined in claim 2 including a well headcomprising means to seal the annulus between said first and said secondcasings, a packing gland to seal the annulus between said second casingand said surface casing, a packing gland to seal the annulus betweensaid surface casing and said production casing, a tubing head and aChristmas-tree.
 6. A well for conducting hot fluids through aperma-frost zone, which comprises: a large-diameter surface conductorcemented in the permafrost; a first casing placed concentrically withinsaid surface conductor and cemented therein, said casing extendingsufficiently into the permafrost to provide support for the well in thepermafrost zone; a second casing placed concentrically within said firstcasing and terminating above the bottom of said first casing, saidsecond casing being supported from said first casing at a point near itsbottom; a surface casing placed concentrically within said second casingand extending downwardly through the permafrost zone, said surfacecasing being supported from the bottom of said first casing and saidsurface casing being cemented below the bottom of said first casing; aproduction casing placed concentrically within said surface casing andextending downwardly to the producing strata, said casing beingsupported from said surface casing at a point within the well and saidcasing being cemented below said surface casing; production tubing toconvey fluids from the producing strata to the surface or vice versa,said tubing being insulated through the permafrost zone and said tubingbeing supported from the production casing within said well; and meansfor refrigerating the interior of said first casing.
 7. The apparatusdefined in claim 6 wherein the annulus between said first and saidsecond casings is closed at its bottom by a packer and wherein saidmeans to refrigerate said first casing includes a small diameter conduitfastened to said second casing and extending substantially its length tointroduce refrigerant into said annulus, and means to withdraw vaporizedrefrigerant from the top of said annulus.
 8. The apparatus defined inclaim 6 wherein the annulus between said first and said second casingsis open at the bottom and wherein said means for refrigerating saidfirst casing includes means for circulating refrigerant down the annulusbetween said first and said second casings and up the annulus betweensaid second casing and said surface casing.
 9. The apparatus defined inclaim 6 wherein said first casing extends into said permafrost to adepth of about 350 feet.
 10. The apparatus defined in claim 6 includinga casing head on said first casing, means to seal the annulus betweensaid first and said second casings, a packing gland to seal the annulusbetween said second casing and said surface casing, a casing head onsaid surface casing, a packing gland to seal the annulus between saidsurface casing and said production casing, a tubing head on saidproduction casing, and a Christmas-tree mounted on said tubing head. 11.A well for conducting hot fluids through a permafrost zone, whichcomprises: a large-diameter surface conductor cemented in thepermafrost; a first casing placed concentrically within said surfaceconductor and cemented therein, said casing extending sufficiently intothe permafrost to provide support for the well in the permafrost zone; asecond casing placed concentrically within said first casing andterminating above the bottom of said first casing, said second casingbeing supported from said first casing at a point near its bottom; acasing head on said first casing including means to seal the annulusbetween said first casing and said second casing; a surface casingplaced concentrically within said second casing and extending downwardlythrough the permafrost zone, said surface casing being supported fromthe bottom of said first casing and said surface casing being cementedbelow the bottom of said first casing; a packing gland to seal theannulus between said second casing and said surface casing; a casinghead installed at the top of said surface casing; a production casingplaced concentrically within said surface casing and extEndingdownwardly to the producing strata, said casing being supported fromsaid surface casing at a point within the well and said productioncasing being cemented below said surface casing; a packing gland to sealthe annulus between said surface casing and said production casing; atubing head at the top of said production casing; production tubing toconvey fluids from the producing strata to the surface or vice versa,said tubing being insulated through the permafrost zone and said tubingbeing supported from the production casing within said well; aChristmas-tree mounted on said tubing head; and means for refrigeratingthe interior of said first casing.
 12. The apparatus defined in claim 6wherein the annulus between said first and said second casings is closedat its bottom by a packer and wherein said means to refrigerate saidfirst casing includes a small diameter conduit fastened to said secondcasing and extending substantially its length to introduce refrigerantinto said annulus, and means to withdraw vaporized refrigerant from thetop of said annulus.
 13. The apparatus defined in claim 6 wherein theannulus between said first and said second casings is open at the bottomand wherein said means for refrigerating said first casing includesmeans for circulating refrigerant down the annulus between said firstand said second casings and up the annulus between said second casingand said surface casing.
 14. The apparatus defined in claim 11 whereinsaid first casing extends into said permafrost to a depth of about 350feet.
 15. A method for drilling a well through a permafrost zone, whichcomprises: drilling a large diameter hole into said permafrost to adepth below the bottom of the piles supporting the drilling rig; placinga surface conductor pipe in said hole and cementing said conductortherein; drilling a smaller diameter hole in said permafrost to a depthsufficient to support the well in the permafrost; placing a first casingin said hole and cementing said first casing to the surface; drilling aneven smaller diameter hole to a depth below the bottom of thepermafrost; placing the lower section of a surface casing in said hole,supporting said surface casing from the bottom of said first casing, andcementing said surface casing above the bottom of said first casing;placing a second casing in said first casing and supporting said secondcasing from said first casing; placing the upper section of said surfacecasing in said well; drilling an even smaller diameter hole through theproducing zones; placing a production casing in said well and supportingsaid production casing from said surface casing; cementing saidproduction casing above the bottom of said surface casing; placing aproduction tubing in said well; refrigerating the annulus between saidfirst and said second casings; and placing said well in service.